1. Field of the Invention
The present invention relates to a solid oxide fuel cell and a method of manufacturing the same.
2. Description of the Related Art
A solid oxide fuel cell comprises, as shown in FIG. 1, a first region 8 filled with a fuel gas 6, a second region 9 filled with air or oxygen 7, and a solid oxide fuel cell unit 5 separating these regions. When the unit 5 and the gases in the regions 8 and 9 are heated to a temperature, e.g., 1000.degree.C., above which ionic conduction by oxygen occurs in an electrolyte layer 3 of the unit 5, electromotive force is generated between a fuel electrode layer 2 and an air electrode layer 4, both within the unit 5, whereby the cell generates electrical energy supplied via conductor 10 to load 11. The electrolyte layer 3 is required to possess good oxygen ion conductivity, and gas stopping power which hardly permits gas to pass therethrough.
The gas stopping power is required from such an electrolyte layer because, if part of the fuel gas in the fuel gas region transmits through the electrolyte layer to reach the air or oxygen region, or conversely if part of the oxygen gas reaches the fuel gas region through the layer, there is the risk of a combustion reaction between the fuel gas and the oxygen gas. In such cases, part of the fuel gas will be consumed, becoming unable to participate in the generation of power. This leads to a drop in power generation efficiency.
Another problem is that the heat of combustion causes local heating of the cell, resulting in breakage of the unit, etc.
The electrolyte layer is formed by one of the following methods:
1 A method (CVD-EVD method) in which zirconium chloride and yttrium chloride are transformed into their gaseous state at a high temperature and under a reduced pressure, the gases reacting with water vapor to deposit an electrolyte layer. This method will hereinafter be referred to as "the gaseous phase method".
2 A method in which an electrolyte powder is spray coated to form an electrolyte layer. This method will hereinafter be referred to as "the conventional spray coating method".
Although the known gaseous phase method is capable of obtaining an electrolyte layer having great gas stopping power, the speed at which the layer is formed is as low as to be about one hundredth (1/100) of that in the conventional spray coating method. Furthermore, the first method requires strict reaction conditions consisting of a temperature ranging from 1100.degree. to 1200.degree. C., and a degree of vacuum of about 1 Torr (1 Torr=1 mmHg), hence, it requires a large apparatus. Still further, since the zirconium chloride and the yttrium chloride are used as being mixed with helium gas, while the vapor used is mixed with hydrogen, the required running cost is high, and it is also necessary to assure safety.
Another drawback of the gaseous phase method is that since it involves a process performed under reduced pressure, the apparatus used cannot be an apparatus for continuous production but only an apparatus for batch production. Because production by a batch type apparatus requires a long time to raise or lower the temperature, the production efficiency is low. As a result, the fuel cell units obtained are extremely expensive. Thus, the first method is only scarcely suitable for practical use.
Although the conventional spray coating method is capable of forming an electrolyte layer at high speed, the produced layer has insufficient gas stopping power. As a result, the fuel cell units obtained may suffer from local heating due, for instance, to gas leakage, and end up with a short service life. Thus, the second method also fails to be adequately usable in practice.
The gas stopping power questioned here is evaluated in terms of the gas permeability coefficient P (cm.sup.4 /g.multidot.sec) by using a certain measuring device whose basic construction is shown in FIG. 2.
First, a predetermined pressure is applied to a pressurizing region 16 of the device. Then, the quantity Q of gas 13 (cm.sup.3 /sec) which reaches a discharge region 17 of the device after passing through an electrolyte layer 3 formed on a porous substrate 1 is measured by a gas flowmeter 14.
When the type as well as the temperature of the gas are fixed, the quantity Q (cm.sup.3 /sec) of the transmitted gas changes in inverse proportion to the thickness t (cm) 12 of the electrolyte layer, and in direct proportion to both the difference Pd (g/cm.sup.2) in pressure between the pressurizing region 16 and the discharge region 17, measured by pressure gauge 15 and the area S (cm.sup.2) of the electrolyte layer. On the basis of these facts, the gas permeability coefficient P (cm.sup.4 /g.multidot.sec) can be calculated from the following equation: ##EQU1## The electrolyte layer should have as small a gas permeability coefficient P as possible.